For many years, the animal science industry has tried to increase the efficiency of feed utilization by animals. The ruminant animals are of particular economic importance, and so, necessarily, is the efficiency of the utilization of ruminants' feed.
In the course of investigating the efficiency of feed use, the mechanism by which ruminants digest and degrade the components of their feed to form molecules which can be metabolically utilized has been intensively studied. The mechanism of carbohydrate utilization is now well known. Microorganisms in the rumen of the animal ferment the carbohydrate to produce monosaccharides, and then degrade the monosaccharides to pyruvate compounds.
It is to be understood that the rumen produces 2-carbon, 3-carbon, and 4-carbon compounds in the form of acids and salts and other derivatives of the acids. Since it is impossible to identify precisely what form the various compounds take, those compounds are referred to in the art respectively as acetates, propionates, and butyrates.
Pyruvate is then metabolized by microbiological processes to either acetates or propionates. Two acetate compounds may be combined thereafter, still in the rumen, to form a butyrate. Leng, "Formation and Production of Volatile Fatty Acids in the Rumen," Physiology of Digestion and Metabolism in the Ruminant (Phillipson et al. ed.), Oriel Press, pages 408-10.
The animal can utilize butyrates, propionates, and acetates with differing degrees of efficiency. Utilization of those compounds, which are collectively known as volatile fatty acids (VFA), occurs after absorption from the gut of the animal. Butyrates are utilized most efficiently, and acetates the least efficiently. However, the relative efficiency of use of butyrates is negated by the inefficiency of the manufacture of butyrates, which must be made from acetates in the rumen.
One of the major inefficiencies in the rumen is in the manufacture of acetates. Since they are made by the degradation of pyruvates, each molecule of an acetate which is produced is accompanied by a molecule of methane. Most of the methane produced is lost through eructation. Since a butyrate is made from two acetate molecules, each molecule of the relatively efficiently used butyrates results in the loss to the animal of two molecules of methane, with all of the associated energy.
Thus, the efficiency of carbohydrate utilization can be increased by treatments which cause the animal to produce propionates rather than acetates from the carbohydrates. Further, the efficiency of feed use can be effectively monitored by observing the production and concentration of propionate compounds in the rumen. If the animal is making more propionates, it will be found to be using its feed more efficiently.
The relative efficiency of utilization of the VFA's is discussed by McCullough, Feedstuffs, June 19, 1971, page 19; Eskeland et al., J. Anim. Sci. 33, 282 (1971); and Church et al., Digestive Physiology and Nutrition of Ruminants, vol. 2 (1971), pages 622 and 625.
It has been well established that the efficiency of feed utilization by a ruminant animal can be readily determined by chemical analysis of the fermentation occurring in the rumen. For example, Marco et al., U.S. Pat. 3,293,038 taught the use of alkylated phenols as feed additives for improved feed efficiency. They illustrated an in vitro rumen fermentation test, and in vivo animal feeding studies, which were evaluated by chemical analysis of the rumen contents for acetates and propionates.
O'Connor et al., J. Anim. Sci. 30, 812-18 (1970), reported the results of in vitro rumen fermentation tests on a large number of compounds. German Patent 2,059,407 reported the use of a hemiacetal of chloral and starch as a feed additive which inhibits the formation of methane and produces higher than normal levels of propionic and butyric acids.
Marco et al., U.S. Pat. 3,522,353 taught the use of halogenated acyclic carboxylic acids as feed additives. It was there shown that the compounds produced in vitro increases in propionate production, and also increased feed efficiency in animals fed those compounds. To a similar effect is Erwin et al., U.S. Pat. 3,564,098.
The condition called ketosis is a manifestation of faulty VFA balance, which amounts to a clinical illness. Ruminant animals maintained on a diet which naturally degrades to a high proportion of acetates and low proportion of propionates are likely to suffer from ketosis. Dairy animals are particularly prone to the condition. Under stress, such as the onset of high lactation, insufficient propionates are available. As a result, more acetates are used leading to a high concentration of ketones in the body and especially in the bloodstream. A treatment for ketosis is to feed propionic acid, a precursor of propionic acid, or glucose which tends to metabolize to propionates. Clearly, if the rumen could be caused to produce more propionates than normal from the diet, ketosis incidence could be reduced.